Pachyrhizus erosus (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Pachyrhizus erosus (L.) Urban

Protologue: Symb. Antill. 4: 311 (1905).

Family: Leguminosae

Chromosome number: 2n= 22

Synonyms

• Dolichos erosus L. (1753),
• Pachyrhizus angulatus Rich. ex DC. (1825),
• P. bulbosus (L.) Kurz (1876).

Sometimes erroneously spelled " Pachyrrhizus ".

Vernacular names

• Yam bean (En). Chop suey bean, jicama (Am).
• Dolique bulbeux, pois batate (Fr)
• Indonesia: bengkuang (general), besusu (Javanese), bangkowang (Sundanese)
• Malaysia: sengkuwang, bengkuwang, mengkuwang
• Philippines: sinkamas (Tagalog), kamias (Ilokano)
• Cambodia: pe' kuëk
• Laos: man ph'au
• Thailand: man-kaeo (general), huapaekua (peninsular), man-lao (northern)
• Vietnam: củdậu (northern), sắn (southern).

Origin and geographic distribution

Yam bean originated in Mexico and Central America as far south as Costa Rica. The crop has been known in cultivation in this region from approximately 1000 BC. The yam bean was originally introduced to the Far East by the Spanish via the Acapulco-Manila route, reaching Amboina prior to the end of the 17th Century. P. erosus is presently found in cultivation (or escaped and naturalized) pantropically.

Uses

The tubers are mostly consumed fresh in salads or lightly fried. In Indonesia, the young tubers are sliced and eaten raw in a mixture of immature fruits with a sweet and pungent sauce ("rujak"), or cut into small cubes as an ingredient of fruit cocktails. In Mexico, they are commonly consumed fresh, sliced and sprinkled with chili and lime juice, or cooked in vegetable soups. As the speciality vegetable on the United States market with the most rapidly increasing demand, P. erosus is sold in supermarkets and salad bars, used in vegetable salads and chop suey. Older tubers from plants grown for seed production or rejects are used as fodder for cattle and pigs. A highly digestible starch may be obtained from old tubers. The immature pods are used locally in South-East Asia as a vegetable. The oil from the seed can be used just like cottonseed oil.

The insecticidal compounds rotenon and rotenoids may be extracted from mature seeds. The seeds may also be ground and used as an insecticide or to poison fish with no residual effects. The vegetative parts of the plant may be used as a high protein fodder after the tubers have been harvested, or serve as a green manure.

Production and international trade

The only reliable statistics concerning production area and trade are from Mexico where about 10 000 ha are cultivated annually for export to the United States at prices reaching US$ 2.50 per kg. As the crop is usually either marketed locally or grown directly by consumers (this applies to Mexico as well as South-East Asia and China), few statistics are available. Judging from the readily available tubers in most South-East Asian countries, however, the production area must be considerable.

Properties

Depending on cultivar, 100 g edible portion of 5-8-months-old tubers contains approximately: water 80-90 g, protein 1.0-2.5 g, fat 0.2 g, carbohydrates 10-17 g, fibre 0.5-1.0 g, ash 0.5-1.0 g, vitamin C 18 mg. The energy value averages 197 kJ/100 g. The high quality starch contained in older tubers has a grain diameter of 8-35μm.

Per 100 g edible portion, immature pods contain: water 86 g, protein 2.6 g, fat 0.3 g, carbohydrates 10 g, fibre 2.9 g, ash 0.7 g, vitamin C 27 mg.

Mature seeds contain about 30% fatty oils, 0.5-1.0% rotenon and 0.5-1.0% rotenoids. The fatty oils have a quality comparable to cottonseed oil. The foliage contains less than 0.01% rotenon and rotenoids; the tubers are free of these toxic substances.

The weight of 1000 seeds ranges from 150-250 g, depending on the cultivar.

Description

• Perennial, herbaceous, strigose to hirsute, climbing or trailing vine, 2-6 m long. Roots tuberous, in cultivation one turnip-shaped tuber per plant, in wild types numerous elongated tubers per plant; tubers of cultivars up to 30 cm × 25 cm, skin cream or light to dark brown, flesh white to whitish-yellow.
• Leaves trifoliolate, strigose, with dentate, palmatilobed or entire leaflets; lateral leaflets obliquely rhomboidal to ovate, 2.5-10.5 cm × 2.5-18 cm, 3veined from the deltoid base; terminal leaflet ovate to reniform, 3.5-17.5 cm × 4-21 cm, 3-5-veined from the cuneate base.
• Inflorescence a many-flowered pseudoraceme, up to 55 cm long; pedicel hirsute, up to 1(-2) cm long with caducous and sericeous prophylls.
• Flowers 1-2.5 cm long; calyx brown, hirsute both internally and externally, about 1 cm long, upper lobe formed by 2 adaxial sepals fused almost to the tip, 2.5-7.5 mm long, tube 2.5-6.5 mm long, the 3 lobes acute and shorter than the tube; corolla violet-blue or white; standard, auriculate wings and keel glabrous; stamens about 1-2 cm long, anthers elliptical and dorsifixed; ovary subsessile, multiovulate, with a crenulate disk round its base, style recurved and ventrally ciliated, stigma with subglobose vertical surface.
• Fruit a legume, linear-oblong, 6-13 cm × 8-17 mm, septate between seeds internally, slightly to deeply contracted between seeds externally, strigose to hirsute when young, often glabrous at maturity.
• Seed flat, square to rounded, 3-9 mm × 4-9 mm, olive green to brown or dark reddish brown.
• Seedling with hypogeal germination; first pair of leaves simple and usually entire.

Growth and development

Germination occurs 5-12 days after sowing, depending on cultivar. Flowering is initiated 2-3 months following germination, depending on cultivar and climate; however, since yam bean is shortday sensitive, time of flowering varies with daylength. The flowering period usually lasts 1-2 months. P. erosus is self-pollinated, although 2-4% cross-pollination does occur.

The yam bean is inoculated mainly by Bradyrhizobium strains, but inoculation by Rhizobium strains also occurs. The irregular nodules form clusters (sometimes large) on the lateral roots and occasionally directly on the surface of the tuber; the latter is not desirable.

Tuber growth is initiated about 7 weeks after germination; rapid growth occurs from week 10-15 and coincides with the flowering period. The peak tuber growth is reached after 20 weeks. Marketable tuber size is reached after 4.5-7 months, depending on cultivar and climate. The growing period needed for seed production is 8-10 months, the tubers from these plants are usually of inferior quality.

Other botanical information

In West Java, two different yam bean cultivars are distinguished: "Huwi hiris", with small, sweet tubers and "Bangkowang" with larger tubers; the latter is also cultivated for green manure. In Thailand, both early and late cultivars are grown. Cultivars used in the main production areas in Mexico differ in sap quality: the Nayarit-type tubers have a milky sap, whereas the Guanajuato-type tubers have a watery sap. The latter are preferred by consumers in the United States.

Ecology

Yam bean is quite tolerant of differences in climatic and edaphic conditions, but is generally associated with regions having moderate precipitation (often seasonal) or high precipitation with well-drained soils. In Mexico, it grows up to 1400 m altitude. Optimum daytime temperature level is (21-)24(-28)°C with daylength approaching 12 hours. However, field trials indicate that the nocturnal temperature may be as important in obtaining optimal yields. Both tuber growth and flowering are initiated by decreasing daylength (and night temperature). A well-drained soil, alluvial or volcanic, is preferable, as the crop does not tolerate waterlogging. In areas with high precipitation, cultivation on ridges is recommended.

Propagation and planting

Yam bean is mainly grown from seed. In northern Thailand, yam beans are planted at the beginning of the early monsoon rains in June-July, while in the north-east, the planting is delayed until July-August (if an early cultivar is used) or August-September (when using a late cultivar); in the eastern-central area, planting takes place in November-December in order to obtain the highest economic returns. In central Thailand, the plant densities used are high: 50-120 cm between ridges (or seedbeds), 7-12 cm between rows and 4-12 cm between plants. The seeding rate in Thailand ranges from 60-180 kg/ha. When introducing the crop to a new field, it is recommended to inoculate the seeds with a suitable Bradyrhizobium strain prior to planting.

In the two main areas of cultivation in Mexico, the states Guanajuato and Nayarit situated at the same latitude, the cultivation periods are virtually opposite: in Guanajuato at 1700 m above sea-level, planting takes place in January-March with harvesting in September-November, whereas in Nayarit at 0-100 m altitude, planting takes place in October-November with harvesting following in February-March.

The recommended spacing in Guanajuanto is 80 cm between ridges and 2 rows per ridge with 20 cm between rows and 20 cm between plants; this gives a plant population of 103 000 plants/ha (about 35 kg seed/ha). In Mexico the same plant density is used when growing yam beans either for seed or for commercial tuber production.

Most yam bean in Thailand is cultivated as a sole crop. However, some farmers in the central area intercrop it with coriander (Coriandrum sativum L.). The coriander, used as a culinary herb, is gradually harvested during the first 45 days when the yam bean crop is producing a good ground cover. In West Java, yam bean is often intercropped with staked yard-long bean (Vigna unguiculata (L.) Walp. cv. group Sesquipedalis). Intercropping with maize (Zea mays L.) and common bean (Phaseolus vulgaris L.) is the traditional cultivation practice in Mexico and Central America. The 3 crops are planted simultaneously on ridges, with 2 rows of yam bean (like a commercial sole crop), the common bean (20 cm between plants) between the 2 rows of yam bean on every ridge and the maize (1 m between plants) also between the 2 rows of yam bean, but on every second ridge. The common beans are harvested after about 90 days, yielding about 0.5 t/ha and the maize is harvested after 110-120 days, yielding 1-2 t/ha.

Husbandry

Usually weeding by hand is practised once or twice during the first 1-2 months, until good ground cover has been achieved. When grown on a large scale as a sole crop, weeding is mechanized. Although various reports mention the beneficial effect of trellised cultivation of yam bean on yields, this has not been confirmed in field trials carried out in Tonga (South Pacific) or in Costa Rica. However, when grown for seed production, trellising yam bean facilitates harvesting and may lead to increased seed yield. Reproductive pruning (the removal of fertile shoots) is commonly practised 1-3 times during the flowering period in order to increase tuber size. Thinning is not practised.

The crop is irrigated in some areas. In Thailand overhead irrigation is usually used, while surface irrigation is the common practice in Guanajuanto, Mexico.

Fertilizing is practised in Thailand, usually 180-300 kg/ha, in 1-2 applications during the first 20-30 days after sowing (or just after the first weeding) and the same amount in a later application at the time of the first reproductive pruning. In Mexico, the crop is never fertilized. In some areas in Thailand, a green manure crop is grown prior to yam bean. Both in Thailand and Mexico the crop is usually grown in rotation with maize.

Diseases and pests

A mosaic virus, probably a strain of bean mosaic virus, transmitted by aphids (Aphis spp.) or mealy bugs (Ferrisia virgata) and possibly by seed, results in smaller-sized tubers (yield reductions of 20-30%), discoloured leaves and brittle stems. Occasionally, plants suffering from witches' broom disease have been observed in Tonga, apparently from contamination originating in the indigenous flora.

Several pests have been reported damaging the leaves, the most serious being the rose beetle (Adoretus versutus) in Tonga; Diabrotica spp. have caused extensive damage in some cultivars.

In some parts of Central and South America, various bruchids (Bruchus spp.) cause severe damage to seed while attacking the young and immature pods. Seed yield may be reduced by as much as 70%.

Harvesting

In both Thailand and Mexico, the tubers are lifted manually, usually after using a chisel plough along the ridges (in Thailand) or a "subsoiler" equipped with a crossbar (in Mexico) in order to loosen up the soil.

The marketable size varies according to consumer preferences. In Thailand as in most of South-East Asia, preference is for smaller-sized tubers weighing 0.2-0.4 kg, whereas Mexican and American consumers prefer large-sized tubers of 0.5-2 kg. Old, oversized tubers and rejects are used for livestock feed.

In areas with a seasonal climate, or in irrigated fields where irrigation can be withheld, tubers may be left in the field for up to 3 months after maturity before being lifted. Lifting after heavy rainfall or irrigation is not recommended, especially when temperature is high, as this may cause the tubers to crack or to split.

In Mexico, dried yam bean hay is mixed with maize and lucerne hay (Medicago sativa L.), passed through a roughage mill and fed to livestock.

Crops grown for seed production are generally harvested after 8-10 months, depending on the cultivar. The pods are left to dry in the open when possible until they split open. The use of mechanical shellers is not widespread because, as the seed tends to be somewhat fragile, the percentage of damaged seed is too high.

Yield

Under irrigated conditions, tuber yields range from 40-90 t/ha in Thailand and from 70-100 t/ha in Mexico. Under rainfed conditions yields in Thailand range from 20-40 t/ha and from 40-60 t/ha in Mexico. Maximum yields in experimental fields amount to 80 t/ha (rainfed, Benin), 105 t/ha (rainfed, Tonga), 148 t/ha (rainfed, Costa Rica) and 160 t/ha (surface-irrigated, Mexico). In Tonga, a field trial yielded 72 t/ha (rainfed), giving a dry matter yield of 7.2 t/ha. At 7.4% crude protein this yields 540 kg of crude protein per ha.

Seed production in Thailand ranges from 480-600 kg/ha, in Mexico from 500-1000 kg/ha.

Handling after harvest

After the tubers have been washed and sorted according to size, they can be stored for 1-2 months at a recommended temperature ranging from 12.5-17.5°C at 65-75% relative humidity. The starch content declines to about 2/3 of the value at harvest when stored at 12.5°C for 2 months and to 1/3 after 3 months storage. The level of total soluble sugars rises correspondingly during storage. The rate of water loss is 0.12% of the initial weight per day at 12.5°C and 0.21% per day at 22°C.

Genetic resources

The neotropical genus Pachyrhizus Rich. ex DC. comprises 5 species, of which P. erosus, P. tuberosus (Lamk) Sprengel and P. ahipa (Wedd.) Parodi are cultivated. The germplasm collection at the Department of Botany, Dendrology and Forest Genetics at the Royal Veterinary and Agricultural University, Copenhagen, Denmark, includes some 200 accessions representing all species and more than 400 interspecific hybrids.

Breeding

When selecting new lines, Mexican breeders grow the breeding materials as a commercial crop, i.e. with reproductive pruning at commercial planting density. The best tubers are then selected at harvest and replanted at a much lower planting density in order to produce seed the following season.

Breeding for photothermal-neutral cultivars is currently in progress. This is accomplished by combining the best P. erosus cultivars identified in field trials with the best photothermal-neutral genotypes belonging to either P. tuberosus or P. ahipa. In addition to photothermal neutrality, the resulting interspecific hybrids exhibit a combination of the yield capacity of P. erosus, with either the earliness and bushy erect habit of P. ahipa or the vigour and tolerance to excess precipitation of P. tuberosus. Amazonian P. tuberosus materials in particular possess several attractive agronomic traits for the breeding of new cultivars for the humid tropics. Some of these rare landraces have a higher dry matter content and are used as an attractive and more nutritious alternative to cassava (Manihot esculenta Crantz) by several Amazonian tribes.

Prospects

Yam bean is a promising leguminous tuber crop for the humid tropics. Although already established in many areas of South-East Asia, yam bean might successfully be introduced to many humid zones at present regarded as unsuitable, through breeding efforts involving P. tuberosus.

Literature

• Grum, M., Halafihi, M., Stölen, O. & Sφrensen, M., 1994. Yield performance of yam bean in Tonga, South Pacific. Experimental Agriculture 30: 67-75.
• Schroeder, C.A., 1968. The jicama, a root crop from Mexico. Proceedings of the American Society for Horticultural Science, Tropical Region, 11: 65-71.
• Sφrensen, M., 1988. A taxonomic revision of the genus Pachyrhizus Rich. ex DC. nom. cons. Nordic Journal of Botany 8(2): 167-192.
• Sφrensen, M., Grum, M., Paull, R.E., Vaillant, V., Venthou-Dumaine, A. & Zinsou, C., 1993. Yam bean (Pachyrhizus species). In: Williams, J.T. (Editor): Underutilized crops: pulses and vegetables. Chapman & Hall, London, United Kingdom. pp. 59-102.
• Sφrensen, M. (Editor), 1994. Proceedings of the First International Symposium on Tuber Legumes. Guadeloupe, 21-24 April 1992. Jordbrugsforlaget, Frederiksberg, Denmark. 328 pp.
• Tadera, K., Tanguchi, T., Teremoto, M., Arima, M., Yagi, F., Kobayashi, A., Nagahama, T. & Ishihata, K., 1984. Protein and starch in tubers of winged bean, Psophocarpus tetragonolobus (L.) DC., and yam bean, Pachyrrhizus erosus (L.) Urban. Memoirs Faculty of Agriculture, Kagoshima University, Japan, 20: 73-81.

Authors

M. Sφrensen & W.C.H. van Hoof